1. Field of the Invention
This invention relates generally to a continuous mode dynamic loop compensator, and, more particularly to a compensator that can identify the type of power converter circuit to which it is connected and thereafter maintain system stability by dynamically compensating for system changes that are dependent upon the duty cycle and hence input voltage and output load variations of the converter. Dynamic loop compensation is especially useful in the implementation of Boost and Buck-Boost power converters since it will maintain stability over a wide range of input voltages and output load current variations.
2. Description of the Prior Art
Power converters or dc-dc converters also known as choppers and switching regulators are widely used in electric automobiles, forklifts and dc voltage regulators amongst many other common uses. Basically these converters regulate the output dc voltage by varying the conduction time of a power switching device.
Among these converters there are two basic categories. In the first category, the forward converters, the output current is supplied to the load and to the output filter capacitor while the switching element is closed (on). Then, during the time that the switching element is open (off), the output inductor supplies current to the load and to the output filter capacitor through a recovery diode.
In the second category, stored energy converters, the output inductor stores energy in the form of current while the switching element is closed (on), and the output current is supplied to the load from the output filter capacitor. During the time the switching element is open (off), the output inductor supplies current to the load and to the output filter capacitor through a recovery diode.
Among these two categories there are three main converters circuit designs. These converter circuits determine the magnitude and polarity of the output voltage for a given input voltage. The first converter, the Buck Regulator, is a forward converter in which the average output voltage is less than the input voltage. Second is the Boost Regulator which is a stored energy converter wherein the average output voltage is greater than the input voltage. Finally, the third converter, the Buck-Boost Regulator, is also a stored energy converter in which the output voltage may be either less than or greater in magnitude than the input voltage.
Typically, regulation of the output voltage is achieved using either pulse width or frequency modulation techniques. These modulation techniques are further subdivided into voltage and current modes or a combination of the two depending on the output waveform requirements. In feed-forward voltage mode regulation the input voltage is sensed to modulate the pulse-width or the frequency to maintain proper regulation. Similarly, in feed-forward current mode regulation the input current is sensed as the mechanism used to modulate the pulse-width or the frequency. On the other hand, feedback voltage mode regulation senses the output voltage in determining the pulse-width or the frequency for regulation. Likewise, under feedback current control regulation the output current is sensed to achieve regulation.
Problems associated with these regulators occur when they are configured in the closed loop feedback design. In this case, sufficient loop phase-gain compensation must be applied to the system plant to maintain an adequate phase margin between the input and output voltages thereby preventing oscillations from occurring within the power converter. In the Boost and Buck-Boost converters the problem is further complicated in that changes in the duty cycle dynamically change the location of the poles and zeros of the system plant. In turn, these new pole and zero locations require compensation at different break points. Correspondingly, a fixed compensation network is not the best method of maintaining closed loop stability. However, prior art compensation designs such as lag and lead compensators employ fixed compensation break points and restrict themselves to the sensitivities of a non-dynamic system. As a result, there is a present need for a dynamic loop compensator that maintains system stability over a wide range of input voltages and output load currents. Furthermore, there is a need for a dynamic loop compensator that can identify the converter circuit design to which it is connected and compensate accordingly.